In magnetic resonance imaging (MRI; Magnetic Resonance Imaging) devices, a target body (generally, a human being) is inserted into a static magnetic field (imaging region) generated by a magnet device, the target body is irradiated with RF pulses. A magnetic resonance signal generated in the target body is received, and a tomographic image for a medical diagnosis is obtained. During this, a gradient coil device generates, in an imaging region on which the target body is placed, pulsed gradient magnetic fields of which intensities linearly vary: in an axial direction of the double hollow circular cylinder shapes of a magnet device (a z axis direction); a lateral direction which is vertical to the z axis direction and in parallel to a floor (x axis direction); and a vertical direction (a y axis direction) which is vertical to the z axis direction, the x axis direction, and the floor, respectively. This adds position information in the target body (imaging region) to the magnetic resonance signal. On the other hand, the gradient coil device generates unnecessary magnetic fields (residual magnetic fields) in a region outside the imaging region. The residual magnetic field generates eddy currents in structures therearound and magnetic fields caused by the eddy currents adversely influence the tomographic image. Accordingly, the MRI device is provided with a shield coil through which a current flows in a direction opposite to that of the main coil in addition to the main coil generating the gradient magnetic field to suppress the residual magnetic field.
The gradient coil generally has a hollow circular cylinder shape having a cross section vertical to the z axis which is a circle and extends along an inner sleeve wall of a magnet device having the double hollow circular cylinder shapes. However, to reduce an oppressive feeling that the target body receives when the target body is inserted into the inner sleeve of the gradient coil, there is a proposal to modify the shape of the circle on the cross section to have an oblong shape to be made closer to a cross section shape of the human body (for example, Patent document 1, etc.). In Patent document 1, a shape on the cross section vertical to the z axis of the main coil and the shield coil of the gradient coil is an oblong shape, i.e., an ellipse shape. Further, in Patent document 1, a gradient coil has been proposed which includes a main coil having an ellipse shape on the cross section and a shield coil, disposed outside the main coil and having a circle on the cross section.
As described above, in the ellipse which is oblong, a major axis of the ellipse is directed to the x direction and a minor axis is directed in the y direction. Accordingly, an x main coil (a major axis gradient coil) for generating a gradient magnetic field in the x direction and a y main coil (a minor axis gradient coil) for generating a gradient magnetic field in the y direction are formed with such an ellipse which is oblong. In this case, because a distance to a center (in the z axis) of the imaging region from a position at a middle of one of a pair of the y main coils is longer than a distance from a position (on the x axis) at a middle of one of a pair of the x main coils, it is necessary to increase a magnetic energy for the x main coils to generate magnetic fields with the x main coils and the y main coils to have the same intensity in the imaging region. In Patent document 2, to solve this problem, the x main coils are arranged inside the y main coils (on a side of the imaging region), and y shields coil (miner axis shield coils) are arranged inside the x shield coils (major axis shield coils) (on a side of the imaging region).
Further, in Patent document 3, to suppress the eddy current to a minimum level, a gradient coil device has been proposed in which a length of the shield coil in the z direction is made longer than a length of the magnet device in the z direction.